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/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _ASM_X86_NOSPEC_BRANCH_H_
#define _ASM_X86_NOSPEC_BRANCH_H_
#include <linux/static_key.h>
#include <linux/objtool.h>
#include <linux/linkage.h>
#include <asm/alternative.h>
#include <asm/cpufeatures.h>
#include <asm/msr-index.h>
#include <asm/unwind_hints.h>
#include <asm/percpu.h>
#include <asm/current.h>
/*
* Call depth tracking for Intel SKL CPUs to address the RSB underflow
* issue in software.
*
* The tracking does not use a counter. It uses uses arithmetic shift
* right on call entry and logical shift left on return.
*
* The depth tracking variable is initialized to 0x8000.... when the call
* depth is zero. The arithmetic shift right sign extends the MSB and
* saturates after the 12th call. The shift count is 5 for both directions
* so the tracking covers 12 nested calls.
*
* Call
* 0: 0x8000000000000000 0x0000000000000000
* 1: 0xfc00000000000000 0xf000000000000000
* ...
* 11: 0xfffffffffffffff8 0xfffffffffffffc00
* 12: 0xffffffffffffffff 0xffffffffffffffe0
*
* After a return buffer fill the depth is credited 12 calls before the
* next stuffing has to take place.
*
* There is a inaccuracy for situations like this:
*
* 10 calls
* 5 returns
* 3 calls
* 4 returns
* 3 calls
* ....
*
* The shift count might cause this to be off by one in either direction,
* but there is still a cushion vs. the RSB depth. The algorithm does not
* claim to be perfect and it can be speculated around by the CPU, but it
* is considered that it obfuscates the problem enough to make exploitation
* extremely difficult.
*/
#define RET_DEPTH_SHIFT 5
#define RSB_RET_STUFF_LOOPS 16
#define RET_DEPTH_INIT 0x8000000000000000ULL
#define RET_DEPTH_INIT_FROM_CALL 0xfc00000000000000ULL
#define RET_DEPTH_CREDIT 0xffffffffffffffffULL
#ifdef CONFIG_CALL_THUNKS_DEBUG
# define CALL_THUNKS_DEBUG_INC_CALLS \
incq %gs:__x86_call_count;
# define CALL_THUNKS_DEBUG_INC_RETS \
incq %gs:__x86_ret_count;
# define CALL_THUNKS_DEBUG_INC_STUFFS \
incq %gs:__x86_stuffs_count;
# define CALL_THUNKS_DEBUG_INC_CTXSW \
incq %gs:__x86_ctxsw_count;
#else
# define CALL_THUNKS_DEBUG_INC_CALLS
# define CALL_THUNKS_DEBUG_INC_RETS
# define CALL_THUNKS_DEBUG_INC_STUFFS
# define CALL_THUNKS_DEBUG_INC_CTXSW
#endif
#if defined(CONFIG_CALL_DEPTH_TRACKING) && !defined(COMPILE_OFFSETS)
#include <asm/asm-offsets.h>
#define CREDIT_CALL_DEPTH \
movq $-1, PER_CPU_VAR(pcpu_hot + X86_call_depth);
#define ASM_CREDIT_CALL_DEPTH \
movq $-1, PER_CPU_VAR(pcpu_hot + X86_call_depth);
#define RESET_CALL_DEPTH \
xor %eax, %eax; \
bts $63, %rax; \
movq %rax, PER_CPU_VAR(pcpu_hot + X86_call_depth);
#define RESET_CALL_DEPTH_FROM_CALL \
movb $0xfc, %al; \
shl $56, %rax; \
movq %rax, PER_CPU_VAR(pcpu_hot + X86_call_depth); \
CALL_THUNKS_DEBUG_INC_CALLS
#define INCREMENT_CALL_DEPTH \
sarq $5, %gs:pcpu_hot + X86_call_depth; \
CALL_THUNKS_DEBUG_INC_CALLS
#define ASM_INCREMENT_CALL_DEPTH \
sarq $5, PER_CPU_VAR(pcpu_hot + X86_call_depth); \
CALL_THUNKS_DEBUG_INC_CALLS
#else
#define CREDIT_CALL_DEPTH
#define ASM_CREDIT_CALL_DEPTH
#define RESET_CALL_DEPTH
#define INCREMENT_CALL_DEPTH
#define ASM_INCREMENT_CALL_DEPTH
#define RESET_CALL_DEPTH_FROM_CALL
#endif
/*
* Fill the CPU return stack buffer.
*
* Each entry in the RSB, if used for a speculative 'ret', contains an
* infinite 'pause; lfence; jmp' loop to capture speculative execution.
*
* This is required in various cases for retpoline and IBRS-based
* mitigations for the Spectre variant 2 vulnerability. Sometimes to
* eliminate potentially bogus entries from the RSB, and sometimes
* purely to ensure that it doesn't get empty, which on some CPUs would
* allow predictions from other (unwanted!) sources to be used.
*
* We define a CPP macro such that it can be used from both .S files and
* inline assembly. It's possible to do a .macro and then include that
* from C via asm(".include <asm/nospec-branch.h>") but let's not go there.
*/
#define RETPOLINE_THUNK_SIZE 32
#define RSB_CLEAR_LOOPS 32 /* To forcibly overwrite all entries */
/*
* Common helper for __FILL_RETURN_BUFFER and __FILL_ONE_RETURN.
*/
#define __FILL_RETURN_SLOT \
ANNOTATE_INTRA_FUNCTION_CALL; \
call 772f; \
int3; \
772:
/*
* Stuff the entire RSB.
*
* Google experimented with loop-unrolling and this turned out to be
* the optimal version - two calls, each with their own speculation
* trap should their return address end up getting used, in a loop.
*/
#ifdef CONFIG_X86_64
#define __FILL_RETURN_BUFFER(reg, nr) \
mov $(nr/2), reg; \
771: \
__FILL_RETURN_SLOT \
__FILL_RETURN_SLOT \
add $(BITS_PER_LONG/8) * 2, %_ASM_SP; \
dec reg; \
jnz 771b; \
/* barrier for jnz misprediction */ \
lfence; \
ASM_CREDIT_CALL_DEPTH \
CALL_THUNKS_DEBUG_INC_CTXSW
#else
/*
* i386 doesn't unconditionally have LFENCE, as such it can't
* do a loop.
*/
#define __FILL_RETURN_BUFFER(reg, nr) \
.rept nr; \
__FILL_RETURN_SLOT; \
.endr; \
add $(BITS_PER_LONG/8) * nr, %_ASM_SP;
#endif
/*
* Stuff a single RSB slot.
*
* To mitigate Post-Barrier RSB speculation, one CALL instruction must be
* forced to retire before letting a RET instruction execute.
*
* On PBRSB-vulnerable CPUs, it is not safe for a RET to be executed
* before this point.
*/
#define __FILL_ONE_RETURN \
__FILL_RETURN_SLOT \
add $(BITS_PER_LONG/8), %_ASM_SP; \
lfence;
#ifdef __ASSEMBLY__
/*
* This should be used immediately before an indirect jump/call. It tells
* objtool the subsequent indirect jump/call is vouched safe for retpoline
* builds.
*/
.macro ANNOTATE_RETPOLINE_SAFE
.Lhere_\@:
.pushsection .discard.retpoline_safe
.long .Lhere_\@
.popsection
.endm
/*
* (ab)use RETPOLINE_SAFE on RET to annotate away 'bare' RET instructions
* vs RETBleed validation.
*/
#define ANNOTATE_UNRET_SAFE ANNOTATE_RETPOLINE_SAFE
/*
* Abuse ANNOTATE_RETPOLINE_SAFE on a NOP to indicate UNRET_END, should
* eventually turn into its own annotation.
*/
.macro VALIDATE_UNRET_END
#if defined(CONFIG_NOINSTR_VALIDATION) && \
(defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_SRSO))
ANNOTATE_RETPOLINE_SAFE
nop
#endif
.endm
/*
* Equivalent to -mindirect-branch-cs-prefix; emit the 5 byte jmp/call
* to the retpoline thunk with a CS prefix when the register requires
* a RAX prefix byte to encode. Also see apply_retpolines().
*/
.macro __CS_PREFIX reg:req
.irp rs,r8,r9,r10,r11,r12,r13,r14,r15
.ifc \reg,\rs
.byte 0x2e
.endif
.endr
.endm
/*
* JMP_NOSPEC and CALL_NOSPEC macros can be used instead of a simple
* indirect jmp/call which may be susceptible to the Spectre variant 2
* attack.
*
* NOTE: these do not take kCFI into account and are thus not comparable to C
* indirect calls, take care when using. The target of these should be an ENDBR
* instruction irrespective of kCFI.
*/
.macro JMP_NOSPEC reg:req
#ifdef CONFIG_RETPOLINE
__CS_PREFIX \reg
jmp __x86_indirect_thunk_\reg
#else
jmp *%\reg
int3
#endif
.endm
.macro CALL_NOSPEC reg:req
#ifdef CONFIG_RETPOLINE
__CS_PREFIX \reg
call __x86_indirect_thunk_\reg
#else
call *%\reg
#endif
.endm
/*
* A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
* monstrosity above, manually.
*/
.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req ftr2=ALT_NOT(X86_FEATURE_ALWAYS)
ALTERNATIVE_2 "jmp .Lskip_rsb_\@", \
__stringify(__FILL_RETURN_BUFFER(\reg,\nr)), \ftr, \
__stringify(nop;nop;__FILL_ONE_RETURN), \ftr2
.Lskip_rsb_\@:
.endm
/*
* The CALL to srso_alias_untrain_ret() must be patched in directly at
* the spot where untraining must be done, ie., srso_alias_untrain_ret()
* must be the target of a CALL instruction instead of indirectly
* jumping to a wrapper which then calls it. Therefore, this macro is
* called outside of __UNTRAIN_RET below, for the time being, before the
* kernel can support nested alternatives with arbitrary nesting.
*/
.macro CALL_UNTRAIN_RET
#if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_SRSO)
ALTERNATIVE_2 "", "call entry_untrain_ret", X86_FEATURE_UNRET, \
"call srso_alias_untrain_ret", X86_FEATURE_SRSO_ALIAS
#endif
.endm
/*
* Mitigate RETBleed for AMD/Hygon Zen uarch. Requires KERNEL CR3 because the
* return thunk isn't mapped into the userspace tables (then again, AMD
* typically has NO_MELTDOWN).
*
* While retbleed_untrain_ret() doesn't clobber anything but requires stack,
* entry_ibpb() will clobber AX, CX, DX.
*
* As such, this must be placed after every *SWITCH_TO_KERNEL_CR3 at a point
* where we have a stack but before any RET instruction.
*/
.macro __UNTRAIN_RET ibpb_feature, call_depth_insns
#if defined(CONFIG_RETHUNK) || defined(CONFIG_CPU_IBPB_ENTRY)
VALIDATE_UNRET_END
CALL_UNTRAIN_RET
ALTERNATIVE_2 "", \
"call entry_ibpb", \ibpb_feature, \
__stringify(\call_depth_insns), X86_FEATURE_CALL_DEPTH
#endif
.endm
#define UNTRAIN_RET \
__UNTRAIN_RET X86_FEATURE_ENTRY_IBPB, __stringify(RESET_CALL_DEPTH)
#define UNTRAIN_RET_VM \
__UNTRAIN_RET X86_FEATURE_IBPB_ON_VMEXIT, __stringify(RESET_CALL_DEPTH)
#define UNTRAIN_RET_FROM_CALL \
__UNTRAIN_RET X86_FEATURE_ENTRY_IBPB, __stringify(RESET_CALL_DEPTH_FROM_CALL)
.macro CALL_DEPTH_ACCOUNT
#ifdef CONFIG_CALL_DEPTH_TRACKING
ALTERNATIVE "", \
__stringify(ASM_INCREMENT_CALL_DEPTH), X86_FEATURE_CALL_DEPTH
#endif
.endm
/*
* Macro to execute VERW instruction that mitigate transient data sampling
* attacks such as MDS. On affected systems a microcode update overloaded VERW
* instruction to also clear the CPU buffers. VERW clobbers CFLAGS.ZF.
*
* Note: Only the memory operand variant of VERW clears the CPU buffers.
*/
.macro CLEAR_CPU_BUFFERS
#ifdef CONFIG_X86_64
ALTERNATIVE "", "verw mds_verw_sel(%rip)", X86_FEATURE_CLEAR_CPU_BUF
#else
/*
* In 32bit mode, the memory operand must be a %cs reference. The data
* segments may not be usable (vm86 mode), and the stack segment may not
* be flat (ESPFIX32).
*/
ALTERNATIVE "", "verw %cs:mds_verw_sel", X86_FEATURE_CLEAR_CPU_BUF
#endif
.endm
#ifdef CONFIG_X86_64
.macro CLEAR_BRANCH_HISTORY
ALTERNATIVE "", "call clear_bhb_loop", X86_FEATURE_CLEAR_BHB_LOOP
.endm
.macro CLEAR_BRANCH_HISTORY_VMEXIT
ALTERNATIVE "", "call clear_bhb_loop", X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT
.endm
#else
#define CLEAR_BRANCH_HISTORY
#define CLEAR_BRANCH_HISTORY_VMEXIT
#endif
#else /* __ASSEMBLY__ */
#define ANNOTATE_RETPOLINE_SAFE \
"999:\n\t" \
".pushsection .discard.retpoline_safe\n\t" \
".long 999b\n\t" \
".popsection\n\t"
typedef u8 retpoline_thunk_t[RETPOLINE_THUNK_SIZE];
extern retpoline_thunk_t __x86_indirect_thunk_array[];
extern retpoline_thunk_t __x86_indirect_call_thunk_array[];
extern retpoline_thunk_t __x86_indirect_jump_thunk_array[];
#ifdef CONFIG_RETHUNK
extern void __x86_return_thunk(void);
#else
static inline void __x86_return_thunk(void) {}
#endif
#ifdef CONFIG_CPU_UNRET_ENTRY
extern void retbleed_return_thunk(void);
#else
static inline void retbleed_return_thunk(void) {}
#endif
extern void srso_alias_untrain_ret(void);
#ifdef CONFIG_CPU_SRSO
extern void srso_return_thunk(void);
extern void srso_alias_return_thunk(void);
#else
static inline void srso_return_thunk(void) {}
static inline void srso_alias_return_thunk(void) {}
#endif
extern void retbleed_return_thunk(void);
extern void srso_return_thunk(void);
extern void srso_alias_return_thunk(void);
extern void entry_untrain_ret(void);
extern void entry_ibpb(void);
#ifdef CONFIG_X86_64
extern void clear_bhb_loop(void);
#endif
extern void (*x86_return_thunk)(void);
#ifdef CONFIG_CALL_DEPTH_TRACKING
extern void call_depth_return_thunk(void);
#define CALL_DEPTH_ACCOUNT \
ALTERNATIVE("", \
__stringify(INCREMENT_CALL_DEPTH), \
X86_FEATURE_CALL_DEPTH)
#ifdef CONFIG_CALL_THUNKS_DEBUG
DECLARE_PER_CPU(u64, __x86_call_count);
DECLARE_PER_CPU(u64, __x86_ret_count);
DECLARE_PER_CPU(u64, __x86_stuffs_count);
DECLARE_PER_CPU(u64, __x86_ctxsw_count);
#endif
#else /* !CONFIG_CALL_DEPTH_TRACKING */
static inline void call_depth_return_thunk(void) {}
#define CALL_DEPTH_ACCOUNT ""
#endif /* CONFIG_CALL_DEPTH_TRACKING */
#ifdef CONFIG_RETPOLINE
#define GEN(reg) \
extern retpoline_thunk_t __x86_indirect_thunk_ ## reg;
#include <asm/GEN-for-each-reg.h>
#undef GEN
#define GEN(reg) \
extern retpoline_thunk_t __x86_indirect_call_thunk_ ## reg;
#include <asm/GEN-for-each-reg.h>
#undef GEN
#define GEN(reg) \
extern retpoline_thunk_t __x86_indirect_jump_thunk_ ## reg;
#include <asm/GEN-for-each-reg.h>
#undef GEN
#ifdef CONFIG_X86_64
/*
* Inline asm uses the %V modifier which is only in newer GCC
* which is ensured when CONFIG_RETPOLINE is defined.
*/
# define CALL_NOSPEC \
ALTERNATIVE_2( \
ANNOTATE_RETPOLINE_SAFE \
"call *%[thunk_target]\n", \
"call __x86_indirect_thunk_%V[thunk_target]\n", \
X86_FEATURE_RETPOLINE, \
"lfence;\n" \
ANNOTATE_RETPOLINE_SAFE \
"call *%[thunk_target]\n", \
X86_FEATURE_RETPOLINE_LFENCE)
# define THUNK_TARGET(addr) [thunk_target] "r" (addr)
#else /* CONFIG_X86_32 */
/*
* For i386 we use the original ret-equivalent retpoline, because
* otherwise we'll run out of registers. We don't care about CET
* here, anyway.
*/
# define CALL_NOSPEC \
ALTERNATIVE_2( \
ANNOTATE_RETPOLINE_SAFE \
"call *%[thunk_target]\n", \
" jmp 904f;\n" \
" .align 16\n" \
"901: call 903f;\n" \
"902: pause;\n" \
" lfence;\n" \
" jmp 902b;\n" \
" .align 16\n" \
"903: lea 4(%%esp), %%esp;\n" \
" pushl %[thunk_target];\n" \
" ret;\n" \
" .align 16\n" \
"904: call 901b;\n", \
X86_FEATURE_RETPOLINE, \
"lfence;\n" \
ANNOTATE_RETPOLINE_SAFE \
"call *%[thunk_target]\n", \
X86_FEATURE_RETPOLINE_LFENCE)
# define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
#endif
#else /* No retpoline for C / inline asm */
# define CALL_NOSPEC "call *%[thunk_target]\n"
# define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
#endif
/* The Spectre V2 mitigation variants */
enum spectre_v2_mitigation {
SPECTRE_V2_NONE,
SPECTRE_V2_RETPOLINE,
SPECTRE_V2_LFENCE,
SPECTRE_V2_EIBRS,
SPECTRE_V2_EIBRS_RETPOLINE,
SPECTRE_V2_EIBRS_LFENCE,
SPECTRE_V2_IBRS,
};
/* The indirect branch speculation control variants */
enum spectre_v2_user_mitigation {
SPECTRE_V2_USER_NONE,
SPECTRE_V2_USER_STRICT,
SPECTRE_V2_USER_STRICT_PREFERRED,
SPECTRE_V2_USER_PRCTL,
SPECTRE_V2_USER_SECCOMP,
};
/* The Speculative Store Bypass disable variants */
enum ssb_mitigation {
SPEC_STORE_BYPASS_NONE,
SPEC_STORE_BYPASS_DISABLE,
SPEC_STORE_BYPASS_PRCTL,
SPEC_STORE_BYPASS_SECCOMP,
};
static __always_inline
void alternative_msr_write(unsigned int msr, u64 val, unsigned int feature)
{
asm volatile(ALTERNATIVE("", "wrmsr", %c[feature])
: : "c" (msr),
"a" ((u32)val),
"d" ((u32)(val >> 32)),
[feature] "i" (feature)
: "memory");
}
extern u64 x86_pred_cmd;
static inline void indirect_branch_prediction_barrier(void)
{
alternative_msr_write(MSR_IA32_PRED_CMD, x86_pred_cmd, X86_FEATURE_USE_IBPB);
}
/* The Intel SPEC CTRL MSR base value cache */
extern u64 x86_spec_ctrl_base;
DECLARE_PER_CPU(u64, x86_spec_ctrl_current);
extern void update_spec_ctrl_cond(u64 val);
extern u64 spec_ctrl_current(void);
/*
* With retpoline, we must use IBRS to restrict branch prediction
* before calling into firmware.
*
* (Implemented as CPP macros due to header hell.)
*/
#define firmware_restrict_branch_speculation_start() \
do { \
preempt_disable(); \
alternative_msr_write(MSR_IA32_SPEC_CTRL, \
spec_ctrl_current() | SPEC_CTRL_IBRS, \
X86_FEATURE_USE_IBRS_FW); \
alternative_msr_write(MSR_IA32_PRED_CMD, PRED_CMD_IBPB, \
X86_FEATURE_USE_IBPB_FW); \
} while (0)
#define firmware_restrict_branch_speculation_end() \
do { \
alternative_msr_write(MSR_IA32_SPEC_CTRL, \
spec_ctrl_current(), \
X86_FEATURE_USE_IBRS_FW); \
preempt_enable(); \
} while (0)
DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp);
DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
DECLARE_STATIC_KEY_FALSE(mds_idle_clear);
DECLARE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
DECLARE_STATIC_KEY_FALSE(mmio_stale_data_clear);
extern u16 mds_verw_sel;
#include <asm/segment.h>
/**
* mds_clear_cpu_buffers - Mitigation for MDS and TAA vulnerability
*
* This uses the otherwise unused and obsolete VERW instruction in
* combination with microcode which triggers a CPU buffer flush when the
* instruction is executed.
*/
static __always_inline void mds_clear_cpu_buffers(void)
{
static const u16 ds = __KERNEL_DS;
/*
* Has to be the memory-operand variant because only that
* guarantees the CPU buffer flush functionality according to
* documentation. The register-operand variant does not.
* Works with any segment selector, but a valid writable
* data segment is the fastest variant.
*
* "cc" clobber is required because VERW modifies ZF.
*/
asm volatile("verw %[ds]" : : [ds] "m" (ds) : "cc");
}
/**
* mds_idle_clear_cpu_buffers - Mitigation for MDS vulnerability
*
* Clear CPU buffers if the corresponding static key is enabled
*/
static __always_inline void mds_idle_clear_cpu_buffers(void)
{
if (static_branch_likely(&mds_idle_clear))
mds_clear_cpu_buffers();
}
#endif /* __ASSEMBLY__ */
#endif /* _ASM_X86_NOSPEC_BRANCH_H_ */